Actually, in the example provided it was one linear feature (the waterway) and one polygon (the forest). However, the effect is usually much more obvious when two actual polygons overlap.
Actually, I think that this is a deliberate (and correct) design choice here. There are areas of the world where trees and water overlap (large swamps, mangroves, etc.). It does actually make sense to show “there are both trees and water here” in those areas.
Where water has been marked by polygons, it’s straightforward for mappers to “not have those polygons overlap” if they actually don’t. What isn’t easy is when the waterway is just linear. Here is an example in Derbyshire in England. The river is just represented by a linear way. It varies in size with the seasons and wouldn’t really make sense to map it as an area. There’s no real break in the trees where the river is, so it wouldn’t make sense to stop the trees one side of the river and start again the other. This results (on OSM Carto) with tree symbols drawn over the river.
An example of a map that doesn’t have “trees in the river” is here (one of mine). That’s the result of a few cartographic decisions:
- The region this map covers doesn’t have many swamps or mangroves or similar, so it’s easy to say “always draw water over trees” - most of the time that’ll be correct.
- The symbol for “broadleaved” is deliberately much smaller and greener, so there are fewer places where it might overlap with anything, and the “forest” and “wood” colours don’t have a tree pattern.
- Waterways are shown narrower than on OSM Carto
- The “broadleaved” pattern isn’t shown at low zoom levels.
One thing that OSM Carto might try (and of course anyone can take a copy of OSM Carto and have a go changing it themselves) is to allow swamps to be shown as now, but always draw linear waterways over trees. I suspect that that would cause some artifacts where linear waterways go through water areas, might it might be worth experimenting with.
Edit: For info, an example where a pond in some trees was not marked as an inner of the trees. That’s arguably a mapping error, but is provided just for comparison.